This invention relates to a process for producing high quality, high molecular weight microcrystalline wax from hydrocracked undewaxed bright stock. The process comprises three steps. In the first step, a hydrocracked undewaxed bright stock is hydrodenitrified using, for example, a sulfided nickel-tin or nickel-molybdenum hydrotreating catalyst having a siliceous or alumina matrix. In the second step, the bright stock, having a reduced catalyst poison content, is hydrofinished using, for example, an unsulfided nickel-tin or palladium hydrotreating catalyst having a siliceous or alumina matrix. In the third step, the waxy oil is solvent dewaxed using a conventional dewaxing solvent such as a mixture of methyl-ethyl-ketone (MEK) and toluene. It has been found that this three-step process produces a high quality, high molecular weight microcrystalline wax.
The first two steps are carried out at unusually low liquid hourly space velocity (LHSV), about 0.25 hr.sup.-1. In the first step, a low LHSV permits the desired hydrodenitrification reaction to proceed at relatively low temperatures. Under these conditions, hydrocracking is minimized. In the second step, a low LHSV permits thorough saturation of aromatics. This sequence of steps provides for the recovery of high molecular weight microcrystalline wax in the third and final step, solvent dewaxing.
New markets continue to expand in demand for petroleum waxes. The varied and growing uses for the petroleum waxes have lifted this material from the by-product class to the product class in operations in some refineries. Waxes derived from petroleum are hydrocarbons of three types: paraffin, semi-microcrystalline, and microcrystalline. The quality and quantity of the wax separated from the crude oil depend on the source of the crude oil and the degree of refining to which it has been subjected prior to wax separation. Paraffin, semimicrocrystalline, and microcrystalline waxes may be differentiated using the refractive index of the wax and its congealing point as determined by ASTM D 938. In addition, petroleum waxes can be distinguished by their viscosities. For example, semi-microcrystalline wax has a kinetic viscosity at 98.9.degree. C. of less than 10 mm.sup.2 /s (=cSt), while microcrystalline wax has a kinetic viscosity at 98.9.degree. C. of greater than or equal to 10 mm.sup.2 /s (=cSt).
Microcrystalline wax, which contains substantial portions of hydrocarbons other than normal alkanes, is the most valuable of the petroleum waxes. It is used in the manufacture of many products such as food containers, waxed papers, coating materials, electrical insulators, candles, etc., and is usually obtained from the highest boiling fraction of a crude oil.
In the manufacture of conventional microcrystalline waxes, the bottoms stream from a vacuum tower or "bright stock" is deasphalted to produce a heavy deasphalted oil which is then extracted to partially remove aromatics. The term "microcrystalline wax" generally refers to deoiled (to less than about 5 wt % oil) wax having a melting point varying from about 140.degree. F. to 180.degree. F. which is recovered from this deasphalted, extracted oil by dewaxing and deoiling. The wax obtained by such a process is characterized by a poor odor, a dark color and it contains aromatic impurities as shown by ultraviolet absorption tests. Thus, the wax must be further refined in order to yield useful products. In the past, microcrystalline wax was contacted with solid absorbent materials such as bauxite or clay to absorb the aromatic compounds therefrom which impart unfavorable properties to the wax.
Accordingly, a process which produces a high quality microcrystalline wax, absent the undesirable properties of poor odor, dark color, and aromatic impurities would be advantageous. It is the principle object of this invention to provide such a process.
Various improvements in the refining of microcrystalline waxes have been made over the years. The most notable of these processes have been directed towards catalytic refining of the wax in the presence of hydrogen, also known as hydrofining. For example, U.S. Pat. No. 3,052,622 discloses taking a crude oil residua and simultaneously deasphalting and extracting the aromatics from it via the Duo-Sol process to obtain a waxy petroleum residue which is then hydrofined by passing the wax, in the presence of hydrogen, over a catalyst of nickel oxide on bauxite. The hydrofined product is then dewaxed via a conventional solvent dewaxing process using toluene and MEK as the dewaxing solvent.
None of these prior art processes have been found, however, to be completely satisfactory. To produce a refined wax that meets U.S. Food and Drug Administration (FDA) standards, the produced waxes must be further refined by contacting with a solid absorbent and then acid treated to achieve the necessary FDA color, odor, and color stability requirements. It is therefore a further objective of this invention to produce a wax that meets FDA standards for color, odor, and color stability, without being further refined by expensive and cumbersome solid desorption methods.